Fiber optic frequency shifters are useful devices for a variety of fiber optic sensor and signal processing applications, such as in fiber optic gyroscopes and the like. It has been shown, for example in U.S. Pat. No. 4,684,215, entitled "Single Mode Fiber Optic Single Sideband Modulator," filed on Nov. 30, 1983, and assigned to the assignee of the present application, that light launched in one propagation mode of a fiber can be coupled to another propagation mode and shifted in frequency by propagating an acoustic wave along the length of the fiber to cause a periodic stress of the fiber. The light is shifted in frequency by an amount equal to the frequency of the acoustic wave. The U.S. Pat. No. 4,684,215 is hereby incorporated herein by reference.
As set forth in the above-referenced patent, if the acoustic wave propagates along the fiber in the same direction as the direction of an optical signal propagating through the fiber, light traveling in a first propagation mode in the fiber at a first phase velocity and at a first frequency is coupled to a second propagation mode at a second phase velocity higher than the first phase velocity and is shifted downward in frequency. Similarly, if the light is originally propagating in the fiber in a faster propagation mode, the light is coupled to a slower propagation mode at a higher frequency. On the other hand, if the acoustic wave propagates along the fiber in a direction opposite the direction of an optical signal propagating through the fiber, light traveling in a slower propagation mode is coupled to a faster propagation mode and is shifted upward in frequency. Similarly, light traveling in a faster propagation mode opposite the direction of propagation of an acoustic wave is coupled to a slower propagation mode and is shifted downward in frequency.
For optimal coupling to occur in the frequency shifter described in the above-referenced patent, the acoustic wavelength, in the direction of propagation of the optical signal through the fiber, is preferably substantially equal to a characteristic of the fiber, referred to as the beat length of the fiber for a given optical signal traveling through the fiber at a specified frequency. As is well known, when light travels through a fiber in more than one propagation mode, the light travels through the fiber at a different phase velocity for each of the different propagation modes. Light traveling in the slower propagation mode travels at a lower phase velocity than light in a faster propagation mode. Thus, a light signal having a fixed frequency will have an effective wavelength in the faster propagation mode longer than it has in the slower propagation mode. As the light propagates down the length of the fiber, a phase difference will thus develop between the light in the two modes. At spatially periodic distances, the light in the two modes will be in phase. The distance between successive locations where the light is in phase is referred to as the beat length of the fiber for the two modes at a specified frequency.
For the devices discussed in the above-mentioned patent, the beat length preferably matches the acoustic wavelength in order to achieve optimal optical coupling of energy between the modes. The light propagating along a fiber in one propagation mode is converted to light propagating in a second propagation mode by applying a periodic, traveling wave, compressive force along a segment of the length of the fiber. A more complete description of this technique is found in "Single-Sideband Frequency Shifting in Birefringent Optical Fiber," W. P. Risk, et al., SPIE Volume 478 - Fiber Optic and Laser Sensors II (1984), pp. 91-97, which is hereby incorporated herein by reference. This article will be referred to as the Risk article.
Background information on frequency shifting by excitation of an acoustic wave along a multimode fiber may be found in the aforementioned U S. patent applications Ser. No. 820,513, now abandoned Ser. No. 909,503 now abandoned and Ser. No. 048,142 now U.S. Pat. No. 4,832,437. These prior patent applications are hereby incorporated herein by reference. The principal characteristics of acousto-optic coupling are now summarized hereinbelow.
It has been shown that acoustic flexural waves propagating along an optical two-mode fiber provide an efficient coupling between the modes. This effect was demonstrated in a frequency shifter configuration, although many other applications are conceivable. These include amplitude modulators, phase modulators, switches and switchable couplers.
A multimode fiber is a fiber which can support more than one spatial propagation mode for an optical signal. The fundamental mode, LP.sub.01, has the slowest phase propagation velocity of the spatial propagation modes. The portion of an optical signal traveling in the LP.sub.01 mode has its optical energy concentrated near the center of the core of the optical fiber. Light traveling in the second order LP.sub.11 mode, or odd mode, has an electrical field amplitude distribution having two maxima displaced away from the center of the core. In a substantially straight, unstressed optical fiber, the LP.sub.01 mode and the LP.sub.11 mode are considered to be orthogonal propagation modes, such that light traveling in one of the modes is not ordinarily coupled to the other mode.
It has been found that if a fiber is bent, a portion of the optical energy entering the bent portion of the fiber in one mode (for instance, the first order LP.sub.01 mode) is coupled to the orthogonal mode (for instance, the second order LP.sub.11 mode) as the optical energy propagates through the bent portion of the fiber. It has been discovered that light propagating along such a fiber may be converted from one mode to the other by applying a static periodic microbending force along a segment of the fibers length which is properly matched to the characteristic length L of the fiber. The distance L, also called the beat length, is calculated as L=2.pi./.DELTA.k, where .DELTA.k is the difference in the propagation constants of the two modes along the fiber.
It has also been found that if an optical frequency is selected to provide a minimum beat length for the first and second order propagation modes, the frequency of the optical signal can be varied substantially above and below the center frequency without causing a significant change in the difference between the propagation constants of the two modes. When an optical fiber is formed into a series of periodic microbends which are spaced by a beat length, the coupling between the two spatial propagation modes of an optical signal traveling through the fiber have a cumulative effect. This effect has been demonstrated in theory in "Bending Effects in Optical Fibers," Henry F. Taylor, Journal of Light Wave Technology, vol. LT-2, pp. 616-633 (1984). This article is hereby incorporated herein by reference.
The propagation of an acoustic wave through a fiber not only causes light to be coupled from one propagation mode to another propagation mode, but also allows a shift in frequency. This is disclosed in detail in the Risk article. While this article discusses coupling between polarization modes in a birefringent fiber, a similar effect has been described for coupling between spatial modes in multimode fibers when an externally applied stress is applied to the fiber. See, for example, the above-referenced U.S. Pat. No. 4,684,215. Thus, an optical signal exiting from one end of the fiber exits at a frequency which is shifted in frequency from the frequency which was input at the first end portion of the fiber. This frequency is equal to the frequency input plus or minus the frequency of the signal applied to the transducer used to generate the acoustic wave. Whether the frequency of the acoustic wave is added to or subtracted from the frequency of the optical input signal is determined by whether the signal is input in the first order mode or the second order mode, and whether or not the optical signal is propagating in the same direction as the propagating microbend.
In the embodiment disclosed in the aforementioned prior U.S. patent application Ser. No. 048,142, a transducer in the form of a horn is positioned adjacent an optical fiber and fused thereto to provide good acoustic contact between the fiber and one end of the horn transducer. In this embodiment, the horn transducer is substantially perpendicular to the propagation axis of the light wave in the optical fiber. This horn transducer is tapered, having a diameter substantially larger at one end than the diameter at the other end. A piece of piezoelectric material is bonded to the distal end, i.e., large diameter end, of the horn. When an electrical signal is applied to the piece of piezoelectric material, this material expands and contracts so as to generate a series of acoustic waveforms propagating through the horn from the distal end (large diameter) to the proximal end (smaller diameter). The acoustic energy of this transducer is coupled directly to the optical fiber to cause movement transverse to the longitudinal axis of the fiber, thus inducing a vibration in the fiber which propagates away from the coupling location as a traveling flexural wave or traveling microbend. The tapered horn acts as an acoustic funnel which concentrates the acoustic energy applied at the distal end of the horn. In this embodiment, the optical fiber also comprises a damper proximate to the location where the transducer is fused to the fiber, to limit the travel of the microbend in the fiber to a single direction. This damper is typically formed of damping material which surrounds the fiber at a location proximate to the location at which the horn contacts the fiber. The damper is usually supported by a support block.